The goal of the proposed research is to develop a single procedure which will induce long-term, out-of-phase synchronized division of cultured mammalian cell lines possessing different mean intermitotic times. A synchronization program involving periodic inhibition of DNA synthesis has been developed. It is anticipated that continuous synchronized division of cells with a variety of mean intermitotic times will be achieved with this program by interrupting the inhibition period with a brief release of the inhibition to prevent death due to unbalanced growth. The optimal lengths of the inhibiton and release periods will be determined to develop a single program for synchronization of cells with two to threefold differences in intermitotic times. Hydroxyurea at a concentration of 0.3 mM will be used to inhibit DNA synthesis, and its effects will be studied in Chinese hamster cells, mouse L cells, HeLa cells and human lymphoid cell lines grown in culture at a variety of rates. When simultaneous, out-of-phase synchronous growth of these cultures has been achieved with the program, the effects of various cytotoxic agents on the synchronized cells will be determined in an effort to inactivate one of the cell lines without destroying the proliferative capacity of the other cell lines.